Many electronic circuits acquire a relatively constant voltage source to operate properly. Such circuits are typically powered by an energy source such as a main power or a battery. The output voltage of these energy sources may fluctuate substantially. To provide a relatively constant voltage, regulator circuits have been developed that convert the voltage of the energy source to a relatively constant voltage.
Generally, a voltage regulator is a simple circuit that provides a low cost control device for small power supplies or other devices having low current ratings. A regulator circuit typically includes a pass transistor coupled to an error amplifier and a base control unit.
The pass transistor in a regulator acts as an adjustable resistor where the voltage difference between the input and the desired output appears across the transistor and causes power losses in the transistor. The desired output may be provided to the transistor by the base control unit which receives input from the error amplifier. The error amplifier may measure output voltage of the transistor against a reference voltage.
Voltage regulators generally suffer a fixed "drop off" voltage induced by the pass transistor. Drop off voltage is the minimum voltage difference between the input and the output voltages of the regulator necessary to maintain output regulation. Accordingly, regulators cannot regulate to the supply voltage.